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Abstract
The segregation behaviour of φ 406 mm IN718 ingot was investigated, and the experimental results indicated that Nb was the most segregated element. Two-stage homogenisation treatment was employed to relieve the extensive microsegregation in the as cast ingot. Historically, it was difficult to characterise the effects of the second stage homogenisation because of the uncertainty in locating the diffusion areas. To solve this problem, a 900°C/1 h treatment (‘Tag’ method) was conducted after homogenisation, which could form δ phase (Nb enriched) to trace the diffusion of Nb. The results revealed that the average composition of δ phase area, quantity of δ phase precipitates and sizes of δ phase precipitates change with the proceeding of homogenisation. Eventually, homogeneous distribution of elements could be achieved by means of two-stage homogenisation treatment (1140°C/60 h+1190°C/30 h).
This project is supported by the Major Program for the Fundamental Research of Shanghai Committee of Science and Technology, China (grant no. 08dj1400402). The authors would like to thank Dr W. R. Sun and Dr L. X. Yu from the Institute of Metal Research for their kind help and useful discussions. Dr J. L. Liu, Y. Hu and Dr Z. G. Zhang from the Analysis and Testing Division of Baosteel Research Institute are appreciated for their help in SEM and EPMA characterisation.